In semiconductor device manufacturing, features and geometric patterns are created on various layers of semiconductor wafers using optical photolithography. Typically, optical photolithography involves projecting or transmitting energy or light through a mask or reticle having a pattern made of optically opaque areas and optically clear areas. Alternatively, a mask or reticle may be reflective rather than transmissive. A phase-shifting mask (PSM) is a type of mask or reticle that uses a phase difference rather than a transmittance difference to generate patterns.
A mask is generally used to pattern an entire wafer at a time, while a reticle is used to pattern a portion of a wafer, e.g., in step-and-repeat projection systems. The term “reticle” as used herein refers to any patterning device having a pattern thereon that may be transferred to the entire surface of a semiconductor wafer, or a portion of a surface of a semiconductor wafer or target.
A prior art reticle 10 used to pattern a target such as a semiconductor wafer is shown in FIG. 1. The reticle 10 may comprise a binary chrome-on-glass mask, for example. A transparent substrate 12 comprising silicon quartz, for example, is provided. An opaque layer 14 is deposited over the substrate 12. The opaque layer 14 typically comprises chrome, for example. The opaque layer 14 is patterned with a desired pattern so that light may pass through transparent regions 16 of the opaque layer 14. The opaque layer 14 of the reticle 10 may be patterned by depositing a photoresist, and patterning the photoresist directly using an electron beam or laser to expose the resist, as examples. The photoresist pattern is then transferred into the opaque layer, e.g., by wet etching.
The reticle 10 may be used to pattern a photoresist layer on a target such as a semiconductor wafer 20, shown in FIGS. 2 and 3. FIG. 2 shows a top view of the wafer 20 and FIG. 3 shows a cross-sectional view of the wafer 20 at 3-3′ of FIG. 2. The wafer 20 may comprise a substrate or workpiece 21 having a material layer 23 disposed on the top surface that will be patterned. A photoresist layer 22 is deposited on the top surface of the wafer 20 over the material layer 23 to be patterned. The photoresist layer 22 is patterned by illuminating the photoresist layer 22 of the wafer 20 with energy, e.g., light, through the reticle 10 of FIG. 1. The photoresist layer 22 is then developed, and portions of photoresist layer 22 are removed, leaving a pattern in the photoresist layer 22 that corresponds with the pattern on the reticle 10, shown in FIG. 1. The optically opaque areas 14 of the reticle 10 block the light, thereby casting shadows and creating dark areas, while the optically clear areas 16 allow the light to pass, thereby creating light areas on the wafer 20. The light areas and dark areas may be projected onto and through an optional lens (not shown), and subsequently onto the photoresist layer 22 of the wafer 20.
When the wafer photoresist layer 22 is developed, exposed areas of the photoresist may be removed, leaving a positive image of the reticle 10 in the photoresist layer 22, e.g., for a positive photoresist. Alternatively, unexposed areas of the wafer photoresist layer 22 may be removed, leaving a negative image of the reticle in the photoresist, e.g., for a negative photoresist (not shown).
The patterned photoresist 22 is then used as a mask to pattern the underlying material layer 23 of the wafer 20. For example, the photoresist 22 may be left in place on the wafer 20 while the wafer 20 is exposed to a dry or wet etchant to remove exposed portions of the material layer 23. The photoresist 22 is removed either in a separate etch step, or at the same time the material layer 23 is etched. The patterned material layer 23 is left remaining over the workpiece 21 top surface. Semiconductor wafers 20 are typically manufactured by the deposition and patterning of multiple layers of insulating, conductive and semiconductive materials, in the manner described above. Another way to form the desired layout on the wafers 20 is to process the lithography and developing process, and then deposit a metal or other material layer over the patterned material layer 23, using a damascene process.
The original image of prior art reticles 10 is typically duplicated on the wafer 20, either in the pattern original size, in a 1× magnification scheme, or alternatively, a 4-5× magnification reduction may be used for projection lithography systems to produce a wafer having a material layer 23 pattern that is ¼ or ⅕ smaller than the reticle 10 pattern, for example. Thus, a one-to-one corresponding relationship exists in prior art reticle 10 patterns and images produced on the wafer 20.
A disadvantage of prior art lithography is that this one-to-one relationship between the reticle 10 and the wafer 20 can result in a reticle defect 18, particularly if the defect is large, inducing a flaw 23a on a wafer 20. Hard defects and/or soft defects can be formed during the manufacturing process or handling of a reticle 10. Soft defects refer to pattern defects that may be removed by cleaning, whereas hard defects generally refer to pattern defects that cannot be removed by a cleaning process. Reticles 10 having relatively large reticle defects 18 are unacceptable because the defect may be transferred to the target 20.
Because reticles 10 are typically expensive and time-consuming to manufacture, attempts are usually made to repair them, rather than scrapping them. Larger hard opaque defects 18 are often removed using a laser to evaporate unwanted material. However, reticle 10 defect inspection and repair are difficult, time-consuming tasks. Also, laser repair of a reticle 10 can damage the reticle substrate, leaving a laser burn and possibly creating a printable defect on the substrate 21. The repair of some reticle 10 defects is often impossible to achieve.